IEEE802.11ac is a wireless local area network (LAN) standard and defines a downlink multi-user multiple input multiple output (DL-MU-MIMO) transmission scheme. DL-MU-MIMO uses a technique called “beamforming” and forms spatially orthogonal beams for multiple terminals. For the beam formation, an access point needs to obtain channel information indicative of downlink channels with the respective terminals and calculate weights on a per-terminal basis in accordance with their respective channels. For this purpose, the access point transmits frames for sounding (e.g., null data packets) to the respective terminals, and receives feedback of the downlink channel information indicative of the downlink channels measured in the terminals. In this manner, the access point obtains the pieces of the downlink channel information of the respective terminals.
Since the channels of the respective terminals are susceptible to variations under the influence of Doppler variation, it is necessary for the access point to periodically carry out sounding and identify the variations in the channels in order to apply optimum weights for the terminals. As a relevant technology, a method is proposed according to which re-sounding is carried out when an elapsed time after the previous sounding exceeds a threshold. This is a simple method because the sounding is periodically carried out at a particular time cycle in response to variations in channels. In normal cases, a possible approach is to assume a certain presupposed channel model (Doppler frequency) and decide on the time cycle of operation. However, if the time cycle of operation is too long, it may happen that an actual variation in a channel takes place too early relative to the operation time cycle, which may lead to degradation in the characteristics. On the other hand, if the operation time cycle is too short, the system overhead becomes large. This may lead to degradation in the communication efficiency (throughput).